THIS INVENTION relates to a method of assessing percentage dot in flexographic or the like printing plates formed of light-transmitting material, (typically transparent material).
It is known to use sheet plastics material for the manufacture of printing plates for certain purposes. In particular, some printing processes take advantage of the flexible nature of plastics materials and utilise so-called “flexographic” printing plates of flexible plastics material. Like printing plates of more conventional materials, such plastics printing plates comprise a printing surface formed by co-planar smooth “high” regions, and intervening “low” regions recessed below the level of said “high” regions. (The use of the term “co-planar” in this context is merely intended to mean that the surfaces in question are effectively flat having regard to the scale of the features concerned. Thus, for example, the “high” smooth surface portions may lie on a notional cylindrical or other curved surface, for example, where the flexographic plate passes around a drum or something of that sort).
In such plastics printing plates, the difficulty arises that it is not easy, from an ordinary visual inspection, either with the naked eye or by means of some optical instrument, to distinguish between the “high portions” which will produce a printing impression, and the “low” portions which will not. However, it is necessary to make such a distinction in order to assess and monitor, for example, manufacture of such printing plates, (which, in the case of flexographic printing plates may be carried out using computer-controlled apparatus to erode selected areas of a blank, instead of using the photographic techniques which are more conventional). In particular, it may be necessary to assess the so-called “percentage dot” in selected areas of such a printing plate, which determines the visual density of the printing in the region concerned.
The term “percentage dot” derives from the conventional half tone printing process in which tones between full black and full white are represented by a two-dimensional array of dots, the tone being determined by the size of the dots in relation to the spaces between dots. Where plastics printing plates are employed, the same principle may be applied, although the elements of the array are not necessarily dots but may be areas of different shapes, depending upon the apparatus used to form the plates or on other factors. The term “percentage dot” as used herein is, accordingly, intended to be understood in this wider sense.